This invention relates generally to control systems for controlling compressor trains, and in particular to control systems for load balancing compressor trains.
Conventional control systems for controlling multiple compressor trains use a station controller for controlling the main process gas variable that acts independently of the unit surge controllers. The station controller typically changes the compressor flows to match the process requirements. However, because the individual compressor trains are typically operationally inefficient due to the preset gains and biases of the station controller, the station controller is unable to dynamically change the preset gains and biases in response to load sharing setpoint changes caused by changing operating conditions. As a result, the station controller is typically unable to position the compressor trains for optimal load sharing. Furthermore, there is usually severe interaction between the surge controllers and the station controller when the export flow into the process drops below the minimum flow needed to keep a compressor on the surge control line. This interaction can cause dynamic process instabilities thereby making it impossible for the station controller to maintain the main process gas variable at a desired operating level.
Conventional approaches to overcoming the problems associated with conventional control systems for multiple compressor trains attempt to equalize the relative distances between the operating points of the compressors and their respective surge control lines in order to provide load sharing. However, these conventional approaches suffer from a number of serious drawbacks. For example, using the compressor operating point as the controlled variable suffers from a number of serious disadvantages. In particular, when the export flow into the process as determined by the station controller is less than the compressor flow, which typically occurs after the compressor operating point reaches the surge control line, the anti-surge recycle valve opens to maintain the compressor flow at the fixed minimum flow needed to avoid surge which results in the loss of the dynamic compressor flow signal to the unit load sharing controller. As a result, the unit load sharing controller is prevented from further decreasing the export flow into the process to allow for small variations in the speed of the unit or in the position of the suction throttle valve position thereby lefting the recycle flow increase in order to decrease the export flow into the process. Consequently, optimal load sharing is not provided and the failure of the load sharing system to meet the process export flow requirements results in a loss of control of the main process gas variable.
The present invention is directed to overcoming one or more of the limitations of existing control systems for load balancing compressor trains.
According to an embodiment of the present invention, a method of controlling the operation of a first compressor having an inlet coupled to a suction line and an outlet coupled to a discharge line and a second compressor having an inlet coupled to the suction line and an outlet coupled to the discharge line is provided that includes determining an operating pressure within the suction line, determining a common coefficient of export flow set point for the first and second compressors as a function of the operating pressure within the suction line, and adjusting the operating points of the first and second compressors using the common coefficient of export flow set point.
According to another embodiment of the present invention, a method of controlling the operation of a first compressor having an inlet coupled to a suction line and an outlet and a second compressor having an inlet coupled to the outlet of the first compressor and an outlet coupled to a discharge line is provided that includes determining an operating pressure within the suction line, determining an operating pressure within the inlet to the second compressor, determining first and second coefficients of export flow set point for the first and second compressors as a function of the operating pressures within the suction line and the inlet to the second compressor, and adjusting the operating points of the first and second compressors using the first and second coefficients of export flow set points.
The present embodiments of the invention provide a number of advantages. For example, by operating each compressor with a sufficient margin from the surge line, safe operation of all of the compressors is ensured. Furthermore, the present embodiments maximize the efficiency of the process by minimizing recycle flows for each of the compressors. In addition, the present embodiments maintain the operating points of all of the compressors at the same margin from the surge control line. Furthermore, the present embodiments determine the coefficient of export flow setpoints as a function, in part, of the suction pressure. In addition, the present embodiments, decouple the surge and unit load sharing controllers thereby avoiding dynamic instabilities and ensuring smooth operation of the compressor trains under changing process conditions. Finally, the present embodiments, provide fully automatic operation and compressor load optimization by implementing an integrated load sharing methodology encompassing multiple compressor trains.